Several methods have been described in the literature for producing demineralized or low-ash coal for fuel and other industrial applications, but none have achieved sustained commercial use. Improved processing methods, based on a better understanding of the underlying science, are required in order to foster a successful adoption of chemical cleaning methods for producing superclean coal and its derivatives.
A process was developed in Germany during the 1940's for removing ash-forming mineral matter from physically cleaned black coal concentrates, involving heating the coal as a paste with aqueous alkali solution, followed by solid/liquid separation, acid washing and water washing steps. Reports on this process (1,2) are the earliest accounts known to us of a practical chemical demineralizing method to which the improvements described here may be related. German practice showed that a demineralized coal with an ash yield of 0.28% could be produced from a physically cleaned feed coal which had an ash yield of 0.8%.
The coal-alkali feed paste was stirred at 40.degree.-50.degree. C. for 30 minutes, then pumped through a heat exchanger to a continuously operable gas-heated tubular reactor in which the paste was exposed to a temperature of 250.degree. C. for 20 minutes, under a pressure of 100-200 atmospheres (10-20 MPa). The reaction mixture was then passed through the heat exchanger previously mentioned, in order to transfer heat to the incoming feed, then cooled further in a water-cooled heat exchanger.
The cooled paste was diluted with softened water, then centrifuged to separate and recover the alkaline solution and the alkalized coal. The latter was dispersed into 5% hydrochloric acid, then centrifuged to recover the acidified coal and spent acid, and redispersed in water. The coal was filtered from this slurry, dispersed again in another lot of water and centrifuged to recover the resulting low-ash coal as a damp solid product.
American (3,4) and Indian (5-7) researchers used broadly similar chemical methods, with variations in processing details, to produce low-ash coals from other feed coals, most of which had much higher starting ash levels than the coals that the Germans used. Another American group (at Battelle) claimed advantages for:
(a) Mixed alkali leachants containing cations from at least one element from Group IA and at least one element from Group IIA of the Periodic Table (8,9).
(b) Filtration or centrifugation of the alkalized coal from the spent alkaline leachant, either at the reaction temperature or after rapid cooling to less than 100.degree. C., in order to minimize the formation of undesired constituents, presumably sodalite or similar compounds (9,10), and
(c) Application of the process to low-rank coals which dissolve in the alkali and which can be reprecipitated at a different pH from the mineral matter, thus allowing separation and selective recovery (11).
Other researchers have studied scientific aspects of alkaline extraction of sulphur and minerals, including the relative merits of different alkalis (12-14). Most American work has been directed more at the removal of sulphur than metallic elements, and the acid treatment step is often omitted. However, an American group (at Alcoa) has chemically cleaned coal to less than 0.1% ash yield, concurrently achieving large reductions and low final concentrations of iron, silicon, aluminium, titanium, sodium and calcium. The aim was to produce very pure coal suitable for conversion into electrode carbon for the aluminium industry. This was achieved by leaching powdered coal with hot aqueous alkaline solution under pressure (up to 300.degree. C.), then successively with aqueous sulphuric acid and aqueous nitric acid at 70.degree.-95.degree. C. (15-16).
The present inventors' investigations have been conducted with Australian coals, which usually contain less sulphur, but often more ash-forming mineral matter than Northern Hemisphere coals. For practical industrial applications, it would usually be necessary to start with feed coals containing more mineral matter than the coal concentrates that the Germans used, and to remove a larger proportion of it by chemical means so as to obtain products of similar purity.
Like the Germans, the present inventors find that sodium hydroxide solution, unmixed with oxides or hydroxides of Group IIA cations, is an adequate alkaline leachant but they recommend using different alkali concentrations, coal/liquid ratios and leaching conditions. The present inventors anticipate practical difficulties in separating alkalized coal from spent alkaline leachant on an industrial scale at the temperatures and pressures used in the alkaline leaching step as claimed by Battelle (8,9), but acknowledge advantages in rapid cooling before separating the solid and liquid components as claimed by Battelle (9,10) but previously practiced by the Germans (1,2). The present inventors recommend specific ways of conducting the leaching, cooling and separating steps in association with other procedures.
Previous investigators have usually experienced difficulties in achieving very low ash levels, except when starting with clean coal concentrates as feed. Having studied the chemical and physical factors in more detail, the present inventors recommend specific methods and processing conditions, especially involving the acidification and washing procedures, in order to minimize the residual mineral matter left in the demineralized product. They have also found, contrary to expectations and to German practice, that the process will demineralize coarse batches (5-10 mm) to about the same extent and at about the same rate as with fine coal of typical pulverized fuel.